A laser exposure apparatus is known which comprises a semi-cylindrical base member, and where a bridge in form of a carriage extends transverse to the base member, said carriage at one side being supported and advanced by an advancing device and at the opposite side resting on a guide. Such a mounting of the bridge is not quite satisfying due to a lack of accuracy. In order to obtain an improved accuracy it is also known to place a screw drive in the longitudinal centre plane of the base member and parallel to said base member, but such an arrangement is not preferred because it is thereby difficult to obtain the necessary room for mounting the exposure material on the base member.